feat(storage): add QueueBatchStateStore and shared batch state helpers

[behinddwalls]

Summary

Why?

BatchStore.GetByQueueAndStates is the storage contract's only query-by-attribute, backed by idx_queue_state — the only secondary index in the entire schema. The storage README's key-value contract flags exactly this shape: a genuinely needed reverse lookup should be a first-class mapping store keyed by the lookup attribute, so any backend (SQL, DynamoDB, Bigtable) can serve it as a primary-key read. This PR adds that store and the shared helpers controllers will migrate onto; speculate/run.go already carries a TODO for this replacement.

What?

• entity.QueueBatchState — an advisory membership record filing an in-queue batch under a (queue, state) bucket, existing from batch creation until the batch exits the queue (through terminal states, hence not "active"). Also adds entity.AllBatchStates() for the future conclude-time sweep.
• storage.QueueBatchStateStore — List(queue, state) / Put / Delete, all idempotent. The MySQL impl is backed by a new queue_batch_state table whose PK (queue, state, batch_id) is the lookup: listing a state bucket is a PK-prefix scan, no secondary index.
• submitqueue/core/batch — the shared protocol primitives: Transition (batch CAS, then Put the new-bucket record before Deleting the old one, so a batch always has at least one record), EnsureRecord (idempotent repair for redelivery skip branches), and ListByStates (scan requested buckets, dedupe, hydrate by key with bounded concurrency, classify by the hydrated authoritative state — a stale record can misplace a batch but never misreport it).
• Contract-suite coverage for the new store and a README pointer beside the ChangeRecord mapping-store example.

Scoped to the foundation only: the store has no callers yet, so there is no runtime behavior change. Follow-ups migrate the six batch-transition sites and the two GetByQueueAndStates readers onto the helpers, add conclude-time record deletion, and then remove GetByQueueAndStates + idx_queue_state.

Test Plan

• ✅ bazel test //submitqueue/entity:all //submitqueue/core/batch:all //submitqueue/extension/storage/...
• ✅ bazel test //test/integration/submitqueue/extension/storage/mysql:go_default_test (real MySQL; includes the new TestStorage_QueueBatchStateRecordLifecycle contract case)
• ✅ make tidy / make gazelle / make fmt / make mocks — all stable (re-running produces no diffs)
• Note: make test failures in orchestrator/controller/batch and controller/cancel are pre-existing — they reproduce at clean HEAD db51df4 in a fresh worktree.

[ubettigole]

Looks good, just one small comment